(perl #131786) avoid a duplicate symbol error on _LIB_VERSION

For -flto -mieee-fp builds, the _LIB_VERSION definition in perl.c and
in libieee conflict, causing a build failure.

The test we perform in Configure checks only that such a variable exists
(and is declared), it doesn't check that we can *define* such a variable,
which the code in pp.c tried to do.

So rather than trying to define the variable, just initialize it during
our normal interpreter initialization.

perl.c

@@ -286,6 +286,14 @@ perl_construct(pTHXx)
     PL_localpatches = local_patches;	/* For possible -v */
 #endif
 
+#if defined(LIBM_LIB_VERSION)
+    /*
+     * Some BSDs and Cygwin default to POSIX math instead of IEEE.
+     * This switches them over to IEEE.
+     */
+    _LIB_VERSION = _IEEE_;
+#endif
+
 #ifdef HAVE_INTERP_INTERN
     sys_intern_init();
 #endif

pp.c

@@ -39,14 +39,6 @@
 extern Pid_t getpid (void);
 #endif
 
-/*
- * Some BSDs and Cygwin default to POSIX math instead of IEEE.
- * This switches them over to IEEE.
- */
-#if defined(LIBM_LIB_VERSION)
-    _LIB_VERSION_TYPE _LIB_VERSION = _IEEE_;
-#endif
-
 static const STRLEN small_mu_len = sizeof(GREEK_SMALL_LETTER_MU_UTF8) - 1;
 static const STRLEN capital_iota_len = sizeof(GREEK_CAPITAL_LETTER_IOTA_UTF8) - 1;